Numerous conditions exist for the human body in which a disease or bodily malfunction have its origin in impaired function of the neuromuscular system, including deficiencies based upon impaired sensory as well as motor activity. In the following the problems addressed by the pre-sent invention will be discussed on the basis of the condition known as hemiplegic drop foot, a condition in which a patient is not able to lift, i.e. dorsiflex, the foot during gait, normally based on an Upper Motor Neurone Lesion (UMNL), for which stroke and head-injuries are by far the more prevalent problems with reported prevelances of 12,000/million for stroke and 20,000/million for head injuries.
Quite often persons who suffer a stroke recover a large amount of function following a period of treatment, but a persistent, long-term disability in approximately 10 to 20% of stroke survivors is UpperMotorNeurone-Drop Foot (UMN-DF). UMN-DF typically involves an inability to dorsiflex the foot during the swing phase of gait (Drop Foot), as well as loss of normal hip and knee flexion, and inability to ‘push-off’ as well as spasticity of the calf muscle group.
An important feature of UMNLs is that electrical excitability of the associated peripheral nerves is still intact, thus facilitating the use of Functional Electrical Stimulation (FES) to restore or enhance gait for some of these cases. As early as in 1961, Liberson and his co-workers proposed application of electrical stimulation (ES) to the common peroneal nerve to correct this condition and using a foot-switch synchronised the application of ES to the swing phase of gait, using a device subsequently referred to as a Peroneal Stimulator (PS) or Drop Foot Stimulator (DFS) [Liberson, W. T., Holmquest, H. J., Scott, D. and Dow, M. (1961) Functional Electrotherapy in stimulation of the peroneal nerve synchronized with the swing phase of gait in hemiparetic patients. Arch Phys Med Rehabil 42, 202-205].
The development of FES-based Drop Foot correction has gone through the following evolutionary stages:    (i) Hard-wired Single-channel Surface Drop Foot Stimulators,    (ii) hard-wired Multi-channel Surface Drop Foot Stimulators,    (iii) hard-wired Single-channel Implanted Drop Foot Stimulators,    (iv) microprocessor-based Surface and Implanted Drop Foot Stimulators,    (v) alternative sensors as replacement for the Foot-switch: (a) artificial sensors, and (b) “natural” sensors.
(i) Hard-wired Single-channel Surface DFS: As indicated above, the first reported use of electrical stimulation for Hemiplegic Drop Foot Correction was in 1961 by Liberson who proposed to elicit dorsiflexion in a hemiplegic foot, synchronised with the swing phase of gait. Liberson's solution, shown in FIG. 1, comprised power and control box (1), a heel-switch (2), when open, during swing, open-circuits the shunt resistor (3), and enables the delivery of stimulus current across the stimulation electrodes (4). The switch when closed, during stance, connects the shunt resistor across the output of the stimulator and no stimulus is delivered to the stimulation electrodes. The delivery of stimulus to the electrodes positioned for stimulation of the common peroneal nerve) is therefore triggered when the heel-switch opens at reel-off and is terminated when the switch closes at heel-strike. The application of stimulus is thus synchronised with the swing phase of gait. This is an example of a hard-wired stimulator, where the functionality of the stimulator is determined by the wiring of the electronic circuitry. The system performed the essential task of inducing dorsiflexion in the subject's hemiplegic foot at the appropriate point in the gait cycle. Clearly, however, the functionality of the system lacked sophistication and delivered stimuli in a crude fashion compared to the natural performance of the foot-lifter neuromuscular system.
(ii) Hard-wired Multi-channel Surface Drop Foot Stimulators: The first group to present a major technical innovation on Liberson's design was Kralj and his co-workers from the University of Ljubljana in Slovenia. They proposed in 1971 the use of multiple channels of stimulation in the drop foot stimulator and a radio link between the heel-switch and the stimulator [Kralj, A., Trnkoczy, A. and Acimovic, R. (1971) Improvement in Locomotion in Hemiplegic Patients with Multichannel Electrical Stimulation. In: Anonymous Human Locomotor Engineering—A review of developments in the field including advances in prosthetics and the design of aids and controls, pp. 45-50. Institute of Mechanical engineers]. The proposed stimulator had three stimulation channels enabling different muscle groups to be controlled independently, such as ankle dorsiflexors and knee flexors and extensors.
(iii) Hard-wired Single-channel Implanted Drop Foot Stimulators: The next major development in hemiplegic drop foot correction technology was the investigation of the possibilities and practicalities of implantable Hemiplegic Drop Foot Stimulators. Jeglic et al., proposed an implanted DFS (IDFS) aimed at overcoming problems of discomfort due to stimulation pain and difficulties experienced by subjects in correctly placing the stimulation electrodes [Jeglic, A., Vanken, E., Benedik, M., “Implantable muscle/nerve stimulator as part of an electronic brace”, in Proc. 3rd International Symposium on External Control of Human Extremities, 1970, pp. 593-603]. Jeglic et al's single channel stimulation device was never successful but may be seen as pre-dating the development of a commercial, implantable, drop foot stimulator by the Rancho Los Amigos Medical Centre/University of Southern California group in California, in conjunction with Medtronic Inc. of Minneapolis [Waters, R. L., McNeal, D. and Perry, J. (1975) Experimental Correction of Foot-drop by Electrical Stimulation of the Peroneal Nerve. J. Bone Joint Surg [A] 57-A (8): 1047-1054].
The three elements of this system, shown in FIG. 2, are: an external module with a transmitting antenna and control module (10), an implanted assembly comprising a receiver, pulse train generator and bipolar electrode (11) and a heel-switch (12) located in the shoe. In common with the Jeglic et al. system, the implanted device required no batteries as electrical power was supplied by electromagnetic induction. The antenna transmitted a radio-frequency signal through the skin and this was taped to the skin directly over the implant. Two incisions were required: one on the medial aspect of the thigh to implant the receiver, another on the lateral aspect of the leg, below the knee, to expose the common peroneal nerve. The system was never put on the market, presumable due to the relatively extensive surgical procedures required.
(iv) Microprocessor-based Surface and Implanted Drop Foot Stimulators: The first use of micro-controller/microprocessor technology for DFS systems is thought to be the incorporation of microprocessor technology into the previously discussed Multi-channel Surface Stimulators.
A primary motivation for the development of such multi-channel implantable stimulators was to overcome the particular problem with single-channel implanted systems reported by Waters et al. [Waters, R. L., McNeal, D., Faloon, W. and Clifford, B. (1985) Functional Electrical Stimulation of the Peroneal Nerve for Hemiplegia—Long-Term Clinical follow-up. J. Bone Joint Surg [A] 67-A (5): 792-793]. Waters et al found that some of the subjects implanted with the Medtronic implanted single-channel DFS system walked with excessive inversion or eversion following surgery. This problem was due to incorrect positioning of the electrodes as the correct placement of the electrode is difficult to determine during surgery. A balanced dorsiflexion response when the subject is prone does not guarantee that the same response will be obtained when the subject is upright, weight-bearing and walking.
A solution to the problem of incorrect electrode placement during surgery, and of the tendency of the electrodes to move post-surgery was proposed by Kelih et al. [Kelih, B., Rozman, J., Stanic, U. and KLjajic, M. (1988) Dual channel implantable stimulator. In: Walling a, W., Boom, H. B. K. and de Vries, J., (Eds.) Electrophysiological Kinesiology, pp. 127-130. Elsevier Science Publichers B. V. (Biomedical Division)]. They proposed a dual-channel implantable stimulator enabling control of two-degrees of freedom of foot movement, viz. dorsiflexion and eversion. Thus, post-surgery, when the subject started to walk using the implant, the stimulus level on each channel could be adjusted to obtain balanced dorsiflexion.
(v) Alternative sensors as replacement for the Foot-switch: Since Liberson's development of the first drop foot stimulator until the early 1990s, the sensor used in FES-based Drop Foot Correction system had been the foot-switch, however, it has been proposed by several researchers, that it would be desirable to replace the foot-switch as the gait sensor in DFS systems for the following reasons:
(1) Fundamentally the traditionally foot-switch has been a contact sensor, requiring repetitive contact/non-contact of the wearer's foot with the foot-switch, which has major implications for the reliability of the sensor. With a DFS system, the ultimate application of the system requires that the subject brings the system home and wears it each day. For the wearer to accept this device and to overcome gadget intolerance the reliability of the system must be high and failure of any component of the system over a short period, including the gait sensor, is unacceptable.
(2) The accepted long-term approach to the implementation of FES-based UMN-DF correction systems is the use of implanted systems. For a completely implanted system, the ability to implant the gait sensor is desirable and the foot-switch is unsuitable for implantation.
(3) Finally the information provided to the DFS system by a foot-switch is very limited, namely, presence or absence of contact by a part of the foot with the ground. This type of signal is quite adequate for the hard-wired DFS systems described, but as the sophistication of DFS systems is increased through the use of more complex control algorithms, the limitation of the foot-switch as a gait sensor should become apparent.
For the reasons outlined, several researchers have evaluated alternative gait sensors using either an artificial gait sensor which would be suitable for implantation or using the body's “natural” sensors. Developments in these two research areas will now be discussed.
(a) Artificial sensors as replacement for the Foot-switch: One of the first groups to propose alternatives to the foot-switch as a gait sensor in DFS systems was Symons et al. at the Rancho Los Amigos Medical Centre/USC [J. Symons, D. McNeal, R. L. Waters, and J. Perry, “Trigger switches for implantable gait stimulation,” in Proc. 9th Annual RESNA Conference, 1986, pp. 319]. Symons carried out preliminary evaluation of an in-house accelerometer fitted to the greater trochanter process of the femur in a vertical orientation to detect the heel strike event. One of the advantages of accelerometers is that they are miniaturised integrated electronic components and as such are highly reliable and therefore very suitable candidates for implantation, which was the rationale for the evaluation of the accelerometer.
Willemsen et al from the University of Twente in the Netherlands, proposed the used of an integrated accelerometer as a replacement for the foot-switch in an UMN-DF correction system [Willemsen, A. T. M., Bloemnof, F. and Boom, H. B. K. (1990) Automatic Stance-Swing Phase Detection from Accelerometer Data for Peroneal Nerve Stimulation. IEEE Transactions Biomedical Engineering 37 (12): 1201-1208]. In their paper, an arrangement of four commercial single-axis accelerometers was placed on the shank of a subject, as shown in FIG. 3. Willemsen et al was able to distinguish between different phases of the gait cycle using the equivalent acceleration at the ankle joint as calculated from four accelerometers placed at locations 30 and 31 and was thus able to detect the onset of swing (push-off) and the termination of swing (heel-strike). Careful attention was paid to the failure rate of detection of push-off and heel-strike. Out of a total of 106 steps, using three hemiplegic subjects, there were errors in only three steps, which is a very good performance.
U.S. Pat. No. 5,814,093 discloses the use of such an artificial sensor arranged corresponding to below the knee of a user. As has been the tradition until now, the (external) stimulation electrodes are placed below the knee joint.
(b) “Natural” sensors as replacement for the Foot-switch: A very elegant and powerful solution to the problems of gait sensors in FES-based UMN-DF correction systems is to use the body's own sensing mechanism. Haugland and Sinkjaer described the use of recordings from a cuff electrode, on the sural nerve, to control the application of stimulus to the common peroneal nerve of a hemiplegic subject [Haugland, M. K. and Sinkjaer, T. (1995) Cutaneous Whole Nerve Recordings Used for Correction for Foot-drop in Hemiplegic Man. IEEE Transactions Biomedical Engineering 3 (4):307-317]. The device is shown in FIG. 4, and consisted of a power and control box 40, a set of surface electrodes 41 for stimulation of the peroneal nerve below the knee, a nerve cuff electrode 42 implanted around the sural nerve 43, having percutaneous wires 44 that with a connector 45 could be connected with the power and control box. To reduce noise in the nerve signal recording an external reference electrode 46 was strapped around the leg. The sural nerve is a sensory nerve which has as it sensory input touch sensors on the lateral part of the foot 47. It was proposed that the conventional heel switch in a DFS system be replaced by a single sural nerve cuff which monitored whether or not the affected foot was supporting weight and used this information to control the application of stimulus in the DFS. Recording nerve signals is referred to as Electroneurography and the corresponding signal is called an ElectroNeuroGram (ENG). It was thus demonstrated that nerve recordings could be used as the basis for the control of a DFS eliminating the need for an external foot-switch and its associated problems.